1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to an apparatus and method for processing AAL2 which supports multiple virtual channels in a mobile communication system, by which a base station, base station controller and switching center of the mobile communication system can transmit/receive data through the multiple virtual channels using AAL2 protocol.
2. Discussion of Related Art
In general, a base station, base station controller or switching center of a mobile communication system transmits data to a destination using the asynchronous transfer mode (ATM). The ATM is a method by which digital data from a user is divided into cells including address information of the destination and a reception number in the header of each cell to create a predetermined fixed-sized (53-byte) cell. This cell is transferred to the destination. In the ATM, the protocol for data transfer consists of a physical layer, an ATM layer, an ATM adaptation layer (AAL) and an upper layer. Particularly, the AAL which reconstructs user application data, i.e. packet data, transmitted from the upper layer into 48 bytes and is classified into AAL1 to AAL5 referring to ITU-T recommendation I series. The AAL2 protocol provides a variable bit rate for packet data and real-time service.
When a user transmits digital information with a low bit rate using ATM, a portion of ATM cell payload is filled with effective data while the remaining portion is padded with “0” to be transferred to a destination. Accordingly, upon application of ATM to a mobile communication system, data items with a low bit rate, such as voice data, transmitted from multiple mobile stations of a same base station are packed in different cells to be transmitted. This wastes cell payload and generates a delay because packet data from a mobile station is not transmitted due to packet data of a short length from another mobile station being packed into a cell to be transferred. The AAL2 protocol is recommended for the purpose of solving these problems and will be explained below.
FIG. 1 illustrates the structure of AAL2 protocol in the related art. Referring to FIG. 1, the AAL2 protocol is divided into a service specific convergence sublayer (SSCS) and a common part sublayer (CPS). Packet data transmitted from a mobile station which is in the form of service data unit (SDU) enters the AAL2 through the service access point (SAP) of the upper layer (not shown). The AAL2 protocol adds a header and trailer to the SDU in its SSCS to generate Service Specific Convergence Sublayer-Protocol Data Unit (SSCS-PDU). Then, the CPS of the AAL2 protocol adds CPS-header to the CPS-PDS (SSCS-PDU in SSCS) to generate a CPS-packet and adds a start field to the CPS-PDU payload to create a 48-byte CPS-PDU. Here, the CPS-packet corresponds to the CPS-PDU payload. The CPS packets of plural users are multiplexed into the CPS-PDU payload because the CPS-PDU is 48-byte.
The CPS-PDU is transferred to the ATM layer where 5-byte destination information is added to the header of the CPS-PDU to generate a 53-byte ATM cell. As described above, the packet data items received from the mobile stations are multiplexed through the AAL2 protocol to be used as the payload of the ATM cell.
FIG. 2 illustrates the structure of CPS-packet data generated in the AAL2 protocol shown in FIG. 1, and FIG. 3 illustrates the structure of CPS-PDU data generated in the AAL2 protocol shown in FIG. 1. Referring to FIGS. 2 and 3, the CPS-packet consists of a CPS-packet header and a CPS-packet payload. The CPS-packet header includes an 8-bit Channel Identifier (CID) field for identifying multiple users in a single virtual channel (VC); a 6-bit Length Indicator (LI) field for indicating the size of the CPS-packet payload; a User-to-User Indication (UUI) field for identifying SSCS data, CPS user and network manager; and a 5-bit Header Error Control (HEC) field for correcting error in the CPS-packet header. Here, a single virtual channel can accommodate 256 users because the CID field is 8-bit. The CPS-PDU consists of an Offset field (OSF) for indicating the length ranging from the starting point of the CPS-PDU payload to the starting point of the next CPS-packet, a start field including a sequence number (SN) and a parity bit (P), a payload (CPS-INFO), and padding field (PAD).
The AAL2 protocol can multiplex packet data items with a short length sent from multiple users on the ATM network into one cell to transmit it. Accordingly, the period of time required for packing every short user data in a new cell can be reduced and the bandwidth of the ATM network can be effectively used. In the conventional AAL2 protocol, however, though transmission of packet data to a destination through a single virtual channel has been supported by the standard recommendation, there has not been a standard recommendation for transmission of packet data (or cell) through multiple virtual channels. Accordingly, it is impossible for multiple mobile stations, base stations or switching centers in a mobile communication system using the AAL2 protocol to transmit packet data through multiple virtual channel, generating a limit in the accommodation capacity for users and the transmission rate. For this reason, there has been a request for standardization for AAL2 protocol supporting multiple virtual channels in the mobile communication system.